Anesthetics and Automaticity in Latent Pacemaker Fibers

Knowledge of arrhythmic or antiarrhythmic actions of anesthetics on automaticity of latent pacemaker fibers has relevance to the intraoperative management of patients with bradyarrhythmia due to sinus node dysfunction or heart block. The authors determined the effects of halothane, enflurane, and isoflurane on automaticity and recovery of automaticity from overdrive suppression in canine Purkinje fibers derived from normal hearts. Purkinje fibers were superfused with a modified Krebs' solution (37 degrees C) containing epinephrine (2 or 15 microM) and equilibrated with a 97% O2-3% CO2 gas mixture (control). Transmembrane action potentials (AP) were recorded using standard microelectrode techniques. Purkinje fibers were then exposed to anesthetics at vaporizer settings of 0.75 or 1.5% (halothane), 1.75 or 3.5% (enflurane), and 1 or 2% (isoflurane), which were equivalent to measured superfusate concentrations of 0.22 or 0.47 mM (halothane), 0.44 or 0.94 mM (enflurane), and 0.28 or 0.53 mM (isoflurane). Compared to control, there was no significant effect of either concentration of the anesthetics on upstroke (phase 0) depolarization, AP amplitude or duration (50% repolarization), or maximum diastolic potential. All three anesthetics increased spontaneous rate. The increase in rate with all three anesthetics was due to enhanced diastolic depolarization (rate dV/dt, phase-4 depolarization). Recovery times from overdrive suppression were determined after 30 or 60 s of pacing at drive cycle lengths of 800, 500, and 400 ms and only at higher anesthetic concentrations. Recovery of automaticity was shortened by halothane only in slowly paced fibers exposed to the lower concentration of epinephrine. Under all other conditions recovery times were not affected by halothane.(ABSTRACT TRUNCATED AT 250 WORDS)